Field of the Invention
The aspect of the embodiments relates to a storage device, an information processing method, and a storage medium.
Description of the Related Art
Recently, devices using an embedded Multi Media Card (eMMC) as a main storage have increased. A NAND flash memory forming the eMMC, in which information is represented by a charge amount in a cell as a minimum storage unit, is roughly classified into different types in accordance with how charges are included. A Single Level Cell (SLC) can store information of 1 bit in a single cell, whereas a Multi Level Cell (MLC) can store information of 2 bits or more in a single cell. Thus, compared with the SLC, the MLC can store a larger amount of information but degrades faster because information based on the amount of charges is required to be accurately determined. For this reason, the MLC has a smaller number of rewritable times, and a shorter data retention period, which is about 10% of that of the SLC. By contrast, compared with the MLC with the same capacity, the SLC can store a smaller amount of information, and is less likely to degrade. Thus, the SLC features a comparatively larger number of rewritable times and a longer data retention period, and is less likely to lose data. In view of the foregoing, some eMMCs have a function of being usable while being divided into one partition corresponding to an MLC mode and another partition to an SLC mode. With this function, each area can be set to be used in the SLC mode or the MLC mode in accordance with how the area is used.
In one method for determining whether data stored in a NAND flash memory is correct and making correction (refreshing), the correction is made with an error correction code (ECC) in a redundant area provided separately from the data. For example, the reliability of the data can be guaranteed by refreshing the entire area of the eMMC once every several months.
This method relies on collective and uniform data refreshing, regardless of whether the partition storing the data corresponds to the MLC (mode) or the SLC (mode) (see Japanese Patent Application Laid-Open No. 2015-148859).